This invention relates generally to vertical form, fill and seal machines, and in particular to a method and apparatus for controlling tension in the film as it passes over the forming shoulder in the forward and reverse directions, without which the film will mistrack over the forming shoulder. The invention allows stripping or settling of product out of the seal area and into a partially formed package to keep any product from contaminating a seal area of the package before the package is sealed, while permitting a high quality package to be manufactured at extremely high speeds.
U.S. Pat. No. 4,288,965 discloses a vertical form, fill and seal machine in which a measuring roll pair is used to precisely meter plastic film from a source thereof, with downstream pull belts being used in conjunction with the measuring rolls to maintain tension over the forming shoulder located between the measuring rolls and the pull belts. This patent does not consider reversing of the measuring rolls and pull belts, and does not address stripping of excess product that may exist in the formed plastic film tube in the area where sealing and severing of successive packages occurs.
Stripping of product into a package made on a vertical form, fill and seal machine has been accomplished for years in order to eliminate any product that is in the formed bag but positioned above the sealing area from contaminating the seal area as a final seal is effected. One such apparatus is set forth in U.S. Pat. No. 4,391,081, assigned to Hayssen Manufacturing Company of Sheboygan, Wisconsin. In this apparatus the product is stripped out of the sealing area and into the bag by using a pair of sliding members which engage the film above horizontally activated sealing dies before the sealing dies close. The sliding members are activated to close and then strip downwardly over the partially formed bag and through the sealing dies prior to the sealing dies closing. This apparatus provides a high quality package but is useful only at speeds below 60 packages per minute. At speeds greater than that, the time required to close and lower the sliding members and strip down is so great that it significantly increases the time required to make a package and thus inhibits productivity.
Other stripping procedures have been used over the years. One such procedure employs a stripping process using the sealing dies, in which the film tube is pulled an amount greater than one package length, the sealing mechanism is then partly closed, and the direction of the film tube is reversed by pulling it in the opposite direction to the correct package length. One version of this method is described in U.S. Pat. No. 3,027,695 assigned to Mira-Pak of Houston, Texas. As in other such systems, this system pulls the film tube to an amount greater than one package length, partially closes the sealing dies, and pulls the tube in the reverse direction through the partially closed sealing dies by moving the entire forming tube assembly in an upward direction, with the film tube therefore moving with the forming tube assembly. This system was used very successfully by the snack food industry during the 1970's. However, this system also suffers problems, including the need of operating at relatively slow speeds, a large moving mass which needs frequent maintenance, and a difficulty of threading the film into the apparatus and then maintaining a constant film tension.
Another stripping system is described in U.S. Pat. No. 4,965,986. This system provides a stripping process controlled by simply reversing the direction of the film tube after the sealing dies have been partly closed. An eccentric mechanism is used to control both the partial closure of the sealing dies as well as their full closure for sealing of the package. The film tube is pulled in a forward direction by conventional pull belts, and is pulled an amount greater than one package length. In stripping, the pull belts are used to pull the film tube in a reverse direction and an auxiliary roll upstream from the forming shoulder pulls the film back over the forming shoulder without moving the forming tube or the sealing dies.
Because some products cannot be stripped into a partially made bag, i.e. powders and fragile products such as cookies, etc., various settling devices operating in the horizontal plane have been evolved. One such device is a "tapper" which may consist of an air cylinder with a rubber bumper, a narrow plate or some other device fixed to its rod end. The cylinder rod is actuated at any time during the bag making and filling cycle, usually below the sealing and severing means and at a frequency of several hundred taps per minute. Another device consists of two rods fastened together at one end similar to a "tuning fork". This device is usually placed below the sealing and severing means with the partially made and filled bag descending through the tines. This device is oscillated horizontally to the incoming partially completed bag with a frequency of several hundred oscillations per minute and an amplitude of up to one inch in either direction. Both these devices shake the partially formed bag in the horizontal direction which in some instances causes the product to settle in the partially formed bag.
Certain products may require both a stripping device and a settling device.